Gas-Phase Acid-Base Properties of Melamine and Cyanuric Acid

The thermochemical properties of melamine and cyanuric acid were characterized using mass spectrometry measurements along with computational studies. A triple-quadrupole mass spectrometer was employed with the application of the extended Cooks kinetic method. The proton affinity (PA), gas-phase basicity (GB), arid protonation entropy (Delta S-p) of melamine were determined to be 226.2 +/- 2.0 kcal/mol, 218.4 +/- 2.0 kcal/mol, and 26.2 +/- 2.0 cal/mol K, respectively. The deprotonation enthalpy (Delta H-acid), gas-phase acidity (Delta(acid)G), and deprotonation entropy (Delta S-acid) of cyanuric acid were determined to be 330.7 +/- 2.0 kcal/mol, 322.9 +/- 2.0 kcal/mol, and 26.1 +/- 2.0 cal/mol K, respectively. The geometries and energetics of melamine, cyanuric acid, and related ionic species were calculated at the B3LYP/6-31+G(d) level of theory. The computationally predicted proton affinity of melamine (225.9 kcal/mol) and gas-phase deprotonation enthalpy of cyanuric acid (328.4 kcal/mol) agree well with the experimental results. Melamine is best represented as the imide-like triazine-triamine form and the triazine nitrogen is more basic than the amino group nitrogen. Cyanuric acid is best represented as the keto-like tautomer and the N-H group is the most probable proton donor. (J Am Soc Mass Spectrom 2010, 21, 1720-1729) (C) 2010 American Society for Mass Spectrometry